The present invention relates to a thin-film two-terminal element, a process for manufacturing the same and a liquid crystal display device, and more particularly, the invention relates to a thin-film two-terminal element aptly usable for office automation equipments, personal computers, portable information terminals and the like, a process for manufacturing the same and a liquid crystal display device provided with the thin-film two-terminal element.
Presently, as liquid crystal display devices used for personal computers or the like, often used are so-called active matrix type liquid crystal display devices in which a switching element such as a TFT (thin film transistor), an MIM (metal-insulator-metal) or the like is provided per pixel on a transparent insulative substrate such as a glass substrate in order to display images of high resolution.
Conventionally, as a semiconductor layer composing a TFT, used is a hydrogenated amorphous silicon (a-Si:H) film formed by a plasma-enhanced CVD method and a polycrystalline silicon (p-Si) or the like which is obtained by re-crystallizing an a-Si film formed by a low pressure CVD method (LPCVD method) or the like by a solid phase epitaxy method by means of thermal treatment or by a laser annealing method.
On the other hand, as a non-linear resistance film composing a MIM element, used is a film of tantalum oxide (Ta2O5) obtained by thermally oxidizing or anodizing tantalum (Ta), aluminum oxide (Al2O3), silicon nitride (SiNx) or the like.
Also, in recent rapid progress of the multimedia society, with development in the field of portable information terminal equipment which allows transfer of information at any place at any time, an increasing importance is attached to small-size, lightweight liquid display devices excellent in portability.
However, the conventional liquid crystal display devices and switching elements as mentioned above have the following problems:
For insulative substrates composing the liquid crystal display devices, usually used are non-alkali glass substrates, quartz substrates or the like as transparent insulative substrates. These substrates establish limits on production of thin, lightweight display devices required for portable information terminal equipment and the like. Also the portable information terminal equipment is likely to often encounter a state of fall, collision or the like during being carried because of its featuring portability. In this respect, the liquid crystal display devices using glass substrates, which are poor in impact resistance, have a problem.
To cope with this, simple matrix-type liquid crystal display devices are being developed which use thin, lightweight plastic substrates superior in impact resistance as insulative substrates (Flat Panel Display, p.p.123-128, 1994).
However, since the simple matrix-type liquid crystal display devices do not have switching elements for respective pixels, they are not suitable for displaying high-definition images and are used for simple display of characters or letters like pagers.
Also, if one tries to mount switching elements such as thin-film transistors, thin-film diodes (TFD, MIM) or the like on a plastic substrate with aim at high-definition display, the manufacture process becomes complicated, especially in the case of using TFTs. Further, since processing at a temperature of about 300xc2x0 C. to 400xc2x0 C. is required, there is a problem in that realization is difficult.
On the other hand, MIM elements, which are thin-film two-terminal elements, have simple element structure in which a non-linear resistance film is sandwiched between electrodes, and the number of production steps is small. Since the processing temperature can be lowered to 200xc2x0 C. or below, it is possible to mount them on a plastic substrate. Active matrix liquid crystal display devices with use of such MIM elements are proposed by Japanese Unexamined Patent Publication Nos. HEI 6(1994)-214220 and HEI 6(1994)-281960, for example.
However, where the MIM elements are intended to be formed on a plastic substrate, there are problems as mentioned below.
First, where a plastic substrate is used as a material for the substrate, there is a problem in that gases are released from the substrate material. That is, since a plastic substrate is more liable to let through gases like vapor as compared with a glass substrate, gases like vapor penetrate from the outside into a liquid crystal layer within a panel after the panel is fabricated, and there occurs deterioration of display or the like.
To cope with this, a gas barrier layer is provided on a face contacting the outside, that is, on a face opposite to a face on which elements are formed, when the liquid crystal panel is fabricated. However, this gas barrier layer prevents penetration of exterior gases but cannot stop release of gases from the plastic substrate itself. Accordingly, during the formation of elements, for example, while a metal film or the like is formed on the substrate, gases released from the substrate are taken in by the film and cause a change in quality of a material for the metal film. As a result, in the case where such the metal film is used for electrode wiring, there occur problems such as a rise in wiring resistance.
As a solution to this, gas barrier layers may be provided on both faces, but in this case, there occur other problems such as an increase in the cost of the substrate.
Generally, for a metal wiring, a Ta film is often used with a view to using an anodized oxide film of the metal wiring as non-linear resistance film. However, since a plastic substrate is less rigid as compared with a glass substrate, a metal wiring of Ta, if formed on a plastic substrate, has a larger stress and causes a warp in the plastic substrate, exfoliation of the metal film and/or the like. There is a problem in that this makes it difficult to continue an element forming process.
In the conventional thin-film two-terminal element, since a first electrode, for example a Ta film, serves also as an interconnect, the Ta film is formed in a thickness of about 300 nm to 500 nm. In this case, the total stress given to the plastic substrate by the Ta film is represented by the following formulae (1) to (3):
Total stress: S="sgr"xc2x7d[N/m]xe2x80x83xe2x80x83(1)
Film stress: "sgr"="sgr"1+"sgr"T[N/m2]xe2x80x83xe2x80x83(2)
Thermal stress: "sgr"T=Ef(xcex1fxe2x88x92xcex1s)xcex94T[N/m2]xe2x80x83xe2x80x83(3)
wherein "sgr"1: inner stress of film, "sgr"T: thermal stress, d: film thickness, Ef: Young""s modulus of thin film, xcex1f: thermal expansion coefficient of thin film, xcex1s: thermal expansion coefficient of substrate and xcex94T: difference in temperature.
From the above formulae, suppose the thickness of a Ta film is 300 nm and a substrate of polyether sulfone (PES) is used as a plastic substrate, the total stress S of the Ta film on the plastic substrate is calculated as about 2,326 [N/m]. Thus, in the case where a Ta film is formed on a plastic substrate to serve also as an interconnect, the substrate deforms because of a large stress of the Ta film, as described in Japanese Unexamined Patent Publication Nos. HEI 6(1994)-214220.
As means of reducing this stress, it is contrived from the above formula to reduce the thickness of the Ta film. However, as a metal interconnect, the resistance of the Ta film becomes greater as the Ta film becomes thinner, which results in problems such as delay of display signals.
As other countermeasure, it can also be contrived to increase the thickness of the plastic substrate or to coat the surface of the substrate in order to provide rigidness.
However, with these measures, there are problems in that the thinness and lightweight, which are merits of plastic substrates, cannot be attained, and furthermore, the cost of substrates rises.
The present invention has been made in view of the above-described problems and an object of the invention is to provide a thin-film two-terminal element capable of being formed on a thin, lightweight substrate of resin excellent in impact resistance which is typified by plastics, a manufacture process therefor and a liquid crystal display device.
According to the present invention, provided is a thin-film two-terminal element comprising a first metal film functioning as a wiring layer and a first electrode; a first insulating film formed on the first electrode of the first metal film and having a non-linear resistance property; a second metal film formed on the first insulating film and functioning as a second electrode; and a third metal film formed in a wiring layer portion of the first metal film and having a smaller stress and a smaller electrical resistance than the first metal film.
Also, according to the present invention, provided is a process for manufacturing a thin-film two-terminal element including the steps of forming a first metal film functioning as a wiring layer and a first electrode on an insulative substrate; forming, at least in a wiring layer portion of the first metal film, a third metal layer having a smaller stress and a smaller electrical resistance than the first metal film; forming a first insulating film having a non-linear resistance property on the first electrode of the first metal film; and forming a second metal film functioning as a second electrode on the insulating film.
Further, according to the present invention, provided is a liquid crystal display device comprising an element side substrate in which a pixel electrode is formed on an insulative substrate provided with the above-described thin-film two-terminal element, the pixel electrode being connected to the second electrode composing the thin-film two-terminal element; an opposite substrate in which a transparent opposite electrode is formed on a second insulative substrate; and a liquid crystal layer sandwiched between the element side substrate and the opposite substrate.
Also, according to the present invention, provided is a thin-film two-terminal element having, formed on a resinous substrate, a first metal film functioning as a wiring layer and a first electrode; a first insulating film formed on the first electrode of the first metal film and having a non-linear resistance property; a second metal film formed on the first insulating film and functioning as a second electrode; and a second insulating film formed under the second metal film expect a portion thereof which electrically functions with the first electrode via the first insulating film.
Further, according to the present invention, provided is a process for manufacturing a thin-film two-terminal element including the steps of forming a first metal film functioning as a first electrode on a resinous substrate; forming a first insulating film having a non-linear resistance property on the first electrode; and forming a second metal film functioning as a second electrode on the first insulating film, wherein, before the second metal film is formed, a second insulating film is formed on the entire surface of the insulative substrate except a portion where the first metal film and the first insulating film are laminated.
Also, according to the present invention, provided is a liquid crystal display device including an element side substrate in which a pixel electrode is formed on the above-described thin-film two-terminal element formed on a resinous substrate, the pixel electrode being connected to the second electrode composing the thin-film two-terminal element; an opposite substrate in which a transparent opposite electrode is formed on a second insulative substrate; and a liquid crystal layer sandwiched between the element side substrate and the opposite substrate.